1. Field of the Invention
The present invention relates to an endoscopic image processor for filtering endoscopic image data properly resolved into a plurality of color signals, and to its image processing method.
2. Description of the Related Art
Endoscopes have come to be widely adopted in recent years. Using an endoscope, a lesion in a body cavity can be observed or, if necessary, treated using a treatment tool merely by inserting the elongated insertion tube into the body cavity without requiring an incision.
Among the aforesaid endoscopes, an optical endoscope (for example, a fiberscope) using an image guide as image transfer means as well as an electronic endoscope (thereafter, electronic scope or scope) using a solid state imaging device, such as a CCD, has been put to practical use.
In another fiberscope, an imaging camera using a solid state imaging device, such as a CCD, is connected to the eyepiece part of the fiberscope so that images can be displayed in color on the monitor.
Moreover, attempts are being made recently to process video signals obtained from this kind of endoscopes in various ways, and thus assist human beings in recognizing things eventually improving the diagnostic capability of the endoscopes.
For example, according to U.S. Pat. No. 4,819,077, an apparatus is disclosed. In the apparatus, red, green and blue (R, G and B) video signals which indicate three primary colors are converted into signals of the hue, saturation and intensity (H,S and I) relatively close to human color perception by performing relatively simple coordinate transformation. Then, the H, S and I signals are enhanced appropriately, then returned to R, G and B signals by performing inverse transformation. Finally, the results are displayed.
Moreover, according to Japanese Patent Laid Open No. 138877/1989, an apparatus is disclosed. In the apparatus, image signals are converted into a Lu* v* color space which is further close to human color perception, then subjected to a variety of enhancement. Then, inverse transformation is carried out.
In Japanese Patent Laid Open No. 26783/1988, the applicant discloses an apparatus which converts video signals into signals in the Lab* b* or Lu* v* color space and enhances intensity L.
In these endoscopes of the prior art, high-frequency components of intensity are enhanced to intensify the contour of a color image or fine patterns. This is because when intensity is disconnected from hue or saturation, enhancement can be done without varying color tone. On the contrary, when each plane of R, G and B signals is enhanced independently, color tone varies resulting in unnatural images.
In endoscopic observation of living bodies, depending on the object to be observed, the endoscope may not provide sufficient observation capability of resolution, brightness and contrast because of mechanical restrictions. In particular, even if an endoscope offering highest resolution available at present is used to observe a fine pattern on the mucosal surface (formed by a glandular cavity of the gastric gland or intestinal gland, an innominate fossa, a capillary, or stein) or an important finding for differentiating a malignant lesion from benign lesions, images having satisfactory quality for diagnosis cannot always be obtained.
Therefore, an endoscope offering higher resolution is greatly desirable. Also, desirable are an image processing procedure which compensates for insufficient contrast or resolution and helps simplify clinical evaluation and an apparatus in which the procedure is implemented.
For example, in ordinary endoscopic images (obtained without using stain), a fine pattern in a mucosal surface is visualized by detecting the variations of G and B signals as variation information forming the pattern. This, presumably, reflects the light absorbing property of hemoglobin in blood.
Images obtained using stain are subject to not only reflection or absorption intrinsic to living bodies but also absorption by stain. These properties determine data variations forming an image.
Conventional high-frequency enhancement in a color space cannot enhance these images successfully. The underlying reason may be as mentioned below.
Data distribution derived from data variations forming a fine pattern is inconsistent with the variation of any one of intensity, hue and saturation in human color perception.
An observer acquires overall information from an image first. Next, he/she observes a fine pattern meticulously in an attempt of acquiring more detailed information. In this stage, the observer concentrates on some information attributable to the living body contained in the image. It has no significant meaning to the observer whether the information is expressed by intensity, saturation or hue.
A well accepted enhancement procedure, which assists an observer, would amplify data variations noted in a region of interest but suppress the other data variations. Enhancement in a color space cannot always satisfy this condition.
Therefore, enhancement in a color space does not always provide an observer of a fine pattern with optimal results.
In addition, data variations in an entire endoscopic image are prone to general variations resulting from lighting or the shape of an object. Intensity enhancement tends to highlight a contour formed by the general variations more clearly. This makes it difficult to observe fine patterns.